The objective of this renewal application is to continue to investigate membrane mechanisms that underly myotonia, a syndrome characterized by abnormal excitability leading to prolonged repetitive discharge of action potentials and abnormal contractions of skeletal muscle fibers. Electrophysiological and pharmacological methods will be used to study myotonic behavior of skeletal muscle fibers from myotonic goats and other naturally myotonic species and in normal skeletal muscle of various species after induction of myotonia by chemical agents, ionic changes, or other treatments. Analsysis will also be made of the action of drugs or of treatments such as water deprivation or denervation which antagonize myotonia. Experiments will be made largely on biopsied muscle fiber bundles of single fibers in vitro. The electrical measurements will include: membrane ionic currents and voltage dependent charge movement with a voltage clamp; membrane excitability and cable parameters with microelectrodes; and monitoring of transverse tubular and other membrane potentials with optical methods employing voltage sensitive dye probes. Initially, the three-vaseline-gap and the three-microelectrode voltage clamps will be used to monitor macroscopic ionic currents, since these are operational in this laboratory. Use of patch-clamp methods is planned for later on in the project to monitor microscopic currents to answer questions concerning channel abnormalities. Extensive use will be made of the laboratory computer for analysis of these data and for testing of hypotheses through computer simulation of myotonic membrane behavior. Particular attention will be paid to the role of chloride conductance channels in the maintenance of normal excitability of skeletal muscle. The ultimate goals of this project are: 1) to increase our basic knowledge of the electrophysiological basis of myotonia at the level of membrane ionic channel function; 2) to provide rational bases for the control of myotonic syndrome in man; and 3) to better understand the frequent association of myotonia with muscular dystrophy.